pvsandiainv SSC module appears to use wrong equation

Hi Paul:

I have been working with the pvsandiainv module from SSC library and, checking the implementation of this module, I found what it is maybe a mistake in pvsandiainv code. If I am not wrong, pvsandiainv uses lib_sandia for ac power calculation and, exactly, sandia_inverter_t. The implemented equation is supposed to be equation 12.3 from the SAM Photovoltaic ModelTechnical Reference Update ( www.nrel.gov/docs/fy18osti/67399.pdf ) but, by checking the source code in  github.com/NREL/ssc/blob/patch/shared/lib_sandia.cpp , I found that the equation is wrong. In line 424, it seems to be a mistake when using C0 instead of C when multipliying by (Pdc[m] - .

Could you please help me in clarifying this?

Thanks in advance.
Kind regards
Miguel

Hi Miguel,

The code in question is here:

github.com/NREL/ssc/blob/5ec04426cdfc4f83c397feb7c1ef66753ab9a70e/shared/lib_sandia.cpp#L424

The original source of that is Equation 1 on p. 14 of King, D.L.; Boyson, W.E.; and Kratochvil, J.A. (2004). Photovoltaic Array Performance Model. 41 pp.; Sandia Report No. 2004-3535. ( PDF 1.8 MB )

It is also described on the Sandia PVPMC website: pvpmc.sandia.gov/modeling-steps/dc-to-ac-conversion/sandia-inverter-model/

And, as you mention as Equation 12.3 on p. 74 of Gilman, P.; Dobos, A.; DiOrio, N.; Freeman, J.; Janzou, S.; Ryberg, D. (2018) SAM Photovoltaic Model Technical Reference Update. 93 pp.;
NREL/TP-6A20-67399. ( PDF 1.8 MB ).

I think there is a discrepancy between SAM and the documented equations. I will investigate and follow up when I learn more.

Best regards,
Paul.

Ok, thank you!

Regards
Miguel

One correction - the source equation is Eq. 1 on pg. 14 of King, D. L., S. Gonzalez, G. M. Galbraith, and W. E. Boyson (2007). Performance Model for Grid-Connected Photovoltaic Inverter. Sandia Report No. SAND2007-5036. September 2007.  Here.

Miguel is correct: the code is in error, multiplying by C0 instead of C from line 416.